Interest in metallocene and non-metallocene single-site catalysts (hereinafter all referred to as single-site catalysts) continues to grow rapidly in the polyolefin industry. These catalysts are more reactive than Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of .alpha.-olefin comonomers, lower polymer density, controlled content and distribution of long-chain branching, and modified melt rheology and relaxation characteristics.
Recent attention has focused on developing improved single-site catalysts in which a cyclopentadienyl ring ligand of the metallocene is replaced by a heteroatomic ring ligand. For example, U.S. Pat. No. 5,554,775 discloses catalysts containing a boraaryl moiety such as boranaphthalene or boraphenanthrene. U.S. Pat. No. 5,539,124 discloses catalysts containing a pyrrolyl ring, i.e., an "azametallocene." In addition, U.S. Pat. No. 5,902,866 discloses azaborolinyl heterometallocenes wherein at least one aromatic ring includes both a boron atom and a nitrogen atom.
U.S. Pat. No. 5,637,660 discloses single-site catalysts that contain a Group 4 transition metal (such as titanium or zirconium) and at least one quinolinyl (or "quinolinoxy") or pyridinyl (or "pyridinoxy") group. In addition to the quinolinoxy or pyridinoxy group, these catalysts contain two "X" ligands, where X is halogen, alkyl, alkoxy, or dialkylamino, and one "L" ligand, where L is X, cyclopentadienyl, substituted cyclopentadienyl, indenyl, or fluorenyl. (Benzyl is not taught as an X or L ligand.) When combined with an activator such as MAO or an ionic borate, these catalysts efficiently polymerize olefins such as ethylene or mixtures of ethylene and .alpha.-olefins. The ready availability of quinolinols and pyridinols and ease of preparation make these catalysts an attractive alternative to other heterometallocenes.
Copending appl. Ser. No. 08/872,659, filed Jun. 10, 1997 as a continuation-in-part of the '660 patent, discloses single-site catalysts that contain a Group 3-10 transition or lanthanide metal and at least one quinolinyl or pyridinyl group. In addition to the quinolinyl or pyridinyl groups, these catalysts include an optional polymerization-stable ligand (such as a cyclopentadienyl group), and at least one "X" ligand, where X can be "halogen, C.sub.1 to C.sub.6 alkyl, C.sub.6 to C.sub.14 aryl, C.sub.7 to C.sub.20 alkaryl, C.sub.7 to C.sub.20 aralkyl, C.sub.1 to C.sub.6 alkoxy, or --NR.sub.2." All of the examples in the '659 application show catalysts that contain one or two quinolinoxy or pyridinoxy groups and two or three chlorides (as "X"). The application gives a skilled person no reason to expect that any one X group (other than, perhaps, chloride) will give a significantly better catalyst than the other listed X groups. For example, a skilled person has no reason to believe that any one ligand within "C.sub.6 to C.sub.14 aryl, C.sub.7 to C.sub.20 alkaryl, C.sub.7 to C.sub.20 aralkyl" will give a significantly better catalyst than the rest.
On the other hand, it would be valuable to identify particular quinolinoxy and pyridinoxy-substituted single-site catalysts with exceptionally high activities. Ideally the catalysts could be prepared inexpensively and in short order, and they would give high yields of olefin polymers.